1. Field of the Invention
This invention relates generally to hobbing machines, and more specifically, to hobbing machines in which finishing operations including gear teeth cutting, deburring and double sided chamfering are performed in several operational steps using a single automated hob spindle.
2. Background Art
Gear teeth are usually cut or milled in automated machines, sometimes referred to as hobbing machines. These machines operate on a usually cylindrical or disc-shaped gear blank by sequentially cutting or milling the gear teeth using a conventional commercially available hobbing machine. Examples of such hobbing machines are described in U.S. Pat. No. 2,451,447, No. 3,130,642 and No. 6,116,828.
Gear teeth are provided as a result of the hobbing operation in an unfinished state, and may result in the teeth having a burr and sharp edges following the initial hobbing operation. Accordingly, a second finishing operation has been found necessary to remove the burr from the cut or milled edges of the gear teeth, and also to bevel the edges to remove the sharp edges and corners. It is generally undesirable to have sharp corners on gear teeth because as gear teeth mesh during gear operation, there is a tendency to stress the contact points of the gear teeth, leading to chipping or other deformation, which events may detract from continued smooth operation of the gears.
There generally exists a need to avoid such sharp corners in gear teeth, especially at the surfaces that initially contact edges and the opposing gear face, since the greatest stresses occur at the contact points. Generally, most hobbing processes and machines provide for a second finishing operation in the course of which the sharp edges are chamfered at an acute angle while simultaneously deburring the burr from the machined edge. In a further improvement, this end chamfer operation has been performed, with respect to the edge having the burrs, in a single operation using a second hob, preferably on the same rotating spindle as the first hob, so that both operations may be performed sequentially without removing the gear blank from hobbing equipment. Such hobbing machines and procedures are described in the aforementioned U.S. Pat. No. 2,451,447, the teachings of which are incorporated by reference herein.
The equipment and operation for providing a chamfer on the gear teeth at one end of a gear produces gear teeth that are capable of, and having sufficient integrity for, use in most applications. However, for those applications in which the stresses acting on the gear teeth are greater than normal, additional operations on the gears are required to enable the gears to withstand the greater stresses. For example, in the aerospace field, gears are usually placed in operation at rotational speeds that are much greater than those normally encountered in, for example, automotive applications. The stresses on the gears as they mesh during operation are magnified and any slight imperfection or defect in the configuration of the gear teeth may cause undesirable chipping or bending stress cracks, ultimately resulting in catastrophic failure of the gears.
One of the problems that contributes to gear teeth deformities is the asymmetrical configuration of the chamfered end of a standard gear made in accordance with the equipment and method of conventional gears, for example, of aforementioned U.S. Pat. No. 2,451,447. The lack of a chamfered or beveled edge surface on both ends of the gear teeth causes unsymmetrical contact and stress forces to act on the gear teeth, and the sharp edges which are left in the gear teeth by the first hobbing operation make the gear teeth more susceptible to cracking, chipping or other deformations at the high stress loads encountered.
Methods have been developed by gear manufacturers to address the problem of asymmetrically chamfered gear teeth. For example, manual chamfering of the back surface of each gear tooth by a person operating a hand grinder is conventionally used. Alternatively, the back edges of the gear teeth are chamfered using a stand alone machine, for example a Gratomat Deburring machine or a Contour Milling machine. Both of these methods have been found to be unsatisfactory, however, because hand operation of a hand grinder even by a skilled operator results in slight imperfections in the uniformity of chamfer, which may themselves lead to asymmetries in the gear teeth, and that may result in the undesirable creation of deformities. More importantly, both of these chamfering methods require a separate operation and separate stand-alone equipment.
These alternative deburring methods are inefficient, mostly unproductive and unnecessarily costly because of the added space requirements, personnel requirements necessary to operate the stand alone separate equipment, and the added process steps required to set up, use and breakdown equipment and mount pieces within a second set of stand alone machinery. Thus, what is considered necessary is equipment and a deburring/chamfering process that enable the production of gears and gear teeth having both a front and rear end chamfer on the gear teeth provided by a single stand-alone unit, that produces gear teeth that are symmetrical so that the gears can withstand the increased stresses of, and can be used in the high rotational speeds found in, aircraft and other aeronautical or aerospace applications.